Officer Grace Gatpandan said police would like to know if they are dealing with a serial phone thief.

On her Facebook page post that shows her in a Supergirl costume, Williams, 34, said she was having a Chinese meal on Tuesday when she noticed a man lurking near her table. Her phone was on the chair next to her and at some point the man grabbed it and left, she said.

Surveillance footage shows Williams leaving the restaurant and confronting the man on the pavement. She said she asked the man if he had accidentally taken the wrong phone and he said he had, before giving it back.

On her Facebook post, Williams said: "My phone was sitting in the chair but I just didn't feel right. He was there too long. 'Is he a customer?' I thought, 'Is he waiting on the bathroom?'. "

The man then grabbed her phone and left the restaurant, she said.

"He began to run but I was too fast. (Those sprints came in handy) I was upon him in a flash!" she wrote.

Danny Snyder, an employee at Mission Chinese Food, where Williams was dining, said the restaurant had never previously had anyone come in and try to steal a phone. He said Williams had eaten at the restaurant before.

Williams adds on her Supergirl post: "Always keep your things close. Fight for what's right. Stand for what you believe in! Be a superhero!"

But Officer Gatpandan said police did not encourage people to go after phone thieves.

"The thing with cellphone snatchers is you don't know who you're dealing with," she said. "You don't know if they're a serial thief. You don't know if they're armed."